Directive antennas



W'. D. LEWIS DIRECTIVE ANTENNAS Aug. 5, 1958 2 Sheets-Sheet 1 Filed June29, 1946 lNl ENTOR M. 0. LE WIS ATTORNEY Aug. 5, 1958 w. D. LEWISDIRECTIVE ANTENNAS Filed Jun 29, 1946 2 Sheets-Sheet 2 mummmvmn STANDINGWAVES DECIBELS m. QQQQNONVQQQ m IEEN ow an 0 cm 9 l PEAK ONE WAY SIGNALSTRENGTH DECIBELS FROM DECIBELS FROM PEAK ONE WAY SIGNAL STRENGTHINVENTOR W 0. LE WIS A 7' TORNEY e e a Fa len 29, 1946, Serial No.686,361

13 Claims. (Cl. 343 333) Application sane This invention relates toantenna particularly to antenna systems for radio devices.

In general, antenna systems for use with directive radio energyreflection devices have employed a primary antenna element and one ormore reflecting elements which present curved surfaces to secureconcentration of the energy distribution. Because surfaces having acylind a1 parabolic curve are more easily and economically manufacturedthan paraboloidally curved surfaces certain advantages accrue to the useof the former surface. Patents 2,434,253 granted on January 13, 1948 toA. J. fleet; and 2,482,162 granted on September 20, 1949 to C. B. H.Feldman each disclose an antenna system comprising a cylindricalparabolic reflector and a wave guide primary antenna. Certaincompensating desirable features have been found to be attendant upon theuse of paraboloidally shaped reflectors. in the copending application ofC. C. Cutler, Serial No. 547,399, filed July 31, 1944 an antenna systemis disclosed for securing uniform illumination, or so-called cosecantcoverage, of plane area. This application matured into Patent 2,489,-865 granted November 29, 1949. Patent 2,427,005 granted on September 9,1947 to A. P. King discloses an antenna system, comprising a concave orparaboloidally shaped reflector and a wave guide or primary antenna, foravoiding the undesirable eflects attendant upon interposing a structureof substantial size in the field of the reflector. ily copendingapplication, Serial No. 574,334, filed January 24, 1945, which maturedinto Patent 2,705,754 granted April 5, 1955, discloses an easilymanufactured cylindrical parabolic reflector system for securing anexceedingly high gain antenna possessing a wide band characteristic anda very sharp major lobe. The system provides a beam possessing adequatecosecant distribution for an early warning or long range searchingsystem.

In addition to the foregoing considerations, as exempiifled by the abovecited applications, most prior art antenna systems employing a we. eguide primary antenna interposed in or adjacent to the axis of theantenna field are subject to detrimental results arising from outgoingenergy being reflected back into the wave guide, or feed line, to causea frequency sensitive impedance mismatch.

Accordingly, it appears advantageous to secure an antenna system thatlends itself to simple economical manufacturing processes, is capable ofa high degree of defini tion in two perpendicular planes, possesses abeam with so-called cosecant coverage suitable for detection of nearbyobjects at considerable elevational angle, and while being of sutficientsize to possess mechanical rug edness does not introduce secondaryreflection or shadow effects and is not susceptible to frequencysensitive mismatches.

lt is one object of the invention to secure a mechanically ruggedantenna system possessing a lobe or beam in which the half power pointlobe width in one plane is at least five times the half power pointwidth in a perpendicular plane.

It is also an object to make possible an asymmetrical systems and moreuse with directive energy distribution whereby the energy in one-half ofthe major lobe in one plane is distributed along a curve approximatingthe curve for a cosecant function.

it is a further object to secure in an antenna system the radiation ofor receipt of energy at high angles of elevation such as would benecessary for the detection of a nearby object in a plane perpendicularto the scanning plane.

It is also an object to make possible in antenna systems possessingconsiderable mechanical ruggedness the radiation and receipt of radioenergy without experiencing secondary reflection effects or withoutdistorting the fields of the antenna.

A still further object of the invention is to make possible thedirective radiation of radio energy while at the same time substantiallyeliminating the reflection of radiated energy back into the primaryantenna.

A still further object is to make possible watertight, pressure proofhousing of the primary antenna element of an antenna system withoutintroducing objectionable reflection effects.

One embodiment of the invention comprises a relatively large cylindricalparabolic main reflector slightly inclined from the vertical in whichthe upper portion is distorted from its cylindrical contour to a slihtly convex contour so that the reflector presents a cylindricalparabolic surface for more than one-half of its transverse dimension anda convex parabolic surface for the remainder. A line type antennaelement, or so-called half pill-box, comprising a relatively smallcylindrical parabolic auxiliary reflector and two parallel end platesattached thereto so as to form therewith a rectangular antenna aperture,faces the main reflector and is so positioned that the short focal line,not greater than one-half wavelength, of the auxiliary reflector isperpendicular to but spaced from the relatively long focal line of thecylindrical parabolic section of the large reflector. A substantiallytransparent dielectric window is positioned in the aforesaid antennaaperture. The plane of the antenna aperture mentioned above issubstantially parallel to and coincident with the focal line of thelarge reflector; however, this plane is at an acute angle to the axialplane of the secondary reflector. A horn is slightly offset from thefocal line of the secondary or auxiliary reflector in such fashion thatno part of the horn is interposed in the useful field of the secondaryreflector.

In operation, waves emanating from the horn appear to originate alongthe focal line of the auxiliary reflector whereby it is uniformlyilluminated. The half pill-box antenna produces a plane wave frontextending perpendicular to the axis or axial plane of the auxiliaryreflector, by reason of the parabolic contour of the auxiliary reflectorand the very small spacing, not greater than a half wavelength, betweenthe parallel end plates. After passing beyond the confines of thereflector and end plates, that is, through the rectangular antennaaperture, the wave tends to take on a circular front in the transverseplane parallel to the short dimension of the rectangular aperture and tothe axial plane. In the longitudinal plane parallel to the longdimension of the rectangular aperture, the Wave front is linear and, asthe plane of the aperture is at an acute angle to the secondaryreflectors axial plane, the wave has a conically shaped wave frontoriginating along the focal line of the large reflector. The main andsecondary reflectors are so disposed with respect to the horizontal andvertical planes that the wave from the secondary reflector describes anacute angle with a perpendicular to the plane of the aperture. As theangle of reflection, or the summation of the reradiated wavelets, isequal to the angle of incidence and the large reflector is inclined fromthe vertical at an angle equal to the angle of incidence it is possibleto locate the secondary reflector beneath the main scanning plane of thesystem. Because the secondary reflector is substantially removed fromthe scanning plane of the large reflector and because the directive hornis substantially removed from the axial field of the secondary reflectorthe amount of outgoing energy that is reflected into the wave guideantenna is comparatively small.

The departure of the upper part of the large reflector from acylindrical to a convex contour provides a refleeting surface along theupper longitudinal dimension of that reflector for high angle radiationand receipt of energy. This makes possible a cosecant distributionpattern for close-in detection of objects at relatively largeelevational angles to the horizontal scanning plane. In reception, thesystem performs in reverse order.

In order to provide a watertight pressure-proof housing for the primaryantenna element the antenna aperture of the secondary reflector isequipped with a dielectric window one-half wavelength in thickness. Thewindow thickness is determined by the operating frequency and thedielectricconstant of the substance forming the window, the optimumthickness being substantially equivalent to one-half wavelength, asmeasured in the aforesaid dielectric substance. The window ismechanically supported on dielectric flanges fitting into slots in theside plates and end members of the reflector. For optimum conditions theslots have a depth in the side plates equal to onehalf the wavelength inthe dielectric used and in the end members equal to one-quarterwavelength in the dielectric. In general, the thickness of the flangesection should be maintained at a minimum consistent with providingadequate mechanical strength to meet the expected stress or pressure towhich the window is likely to be exposed.

The invention will be more readily understood from a study of thefollowing detailed explanation with reference to the accompanyingdrawing in which like reference characters denote elements of similarfunction and in which:

Figs. 1, 2 and 3 are respectively side, front and top views of oneembodiment of the invention;

Fig. 4 is a top cross-sectional view at the line 44 of Fig. 2;

Fig. 5 is a detail partial sectional view of the line antenna elementincluded in the embodiment of Figs. 1, 2 and 3;

Fig. 6 is a perspective view showing the conically shaped contour thewave assumes after it emerges from the line antenna element;

Figs. 7. 8 and 9 are explanatory graphs referred to in the followingexplanation.

Referring to Figs. 1. 2 and 3 reference numeral denotes a translationdevice, such as a radar transcei er. and numeral M denotes a wave guideattached thereto. Numeral 17 denotes a horn extending through the bottom14 of a cylindrical parabolic reflector 19 which has a short focal line20 of a length equal to one-half wavelength or less, an axis or axialplane 23 and a focal plane 38. Numerals 36 and 37 denote metallic endplates attached to each end of the parabolic reflector 1 and formingtherewith a line antenna element 13, or so-called half pill-box, havinga rectangular opening containing the dielectric window 31. Numeral 2.9denotes a larger cylindrical parabolic reflector hereinafter referred tothe main or primary reflector. This reflector has a complex shapecomprising the cylindrical parabolic surface having a transversedimension 46 and a longitudinal dimension 4'7 and a convex parabolicsurface 39 having the same longitudinal dimension 47 and the transversedimension 45. Section 30 is formed by causing the section comprising thedimensions 45, 47 to depart from the straight line of the cylindricalsection essentially along a square law curve, as shown, so that the toplongitudinal edge of the reflector is set back a distance 48 from itsposition in the cylindrical portion of the reflector. Reference numerals26 and 27 denote vertical and horizontal planes respectively. Referenceletter a denotes the angle by which the cylindrical axis of the mainreflector 29, the focal line 23 of this reflector and the plane of theaperture window 31 are inclined from the vertical plane 26. it alsorepresents the angle formed by the plane of the aperture window 31 andthe plane 38 parallel to the latus rectum plane 33 as well as the angleformed by the axial plane 23 and the plane 49 perpendicular to the planeof the window 31.

Referring to Figs. 4 and 5, which illustrate the mounting details forthe dielectric window 33, numeral 3?, denotes a clamping device securedto the outer edge of the side walls 36 and 37 and the two ends of theauxiliary reflector 19 by the machine screws 33. Numerals 3d denotethree inside surfaces of the metallic clamping device 32, the threesurfaces being beveied or flared so as to form impedance improvingflanges and to provide proper illumination of the main reflector 29 bythe line antenna element 18. The side walls and 37 and the end members14 and 19' are grooved or stepped so that, with the clamping device 32in place, receptacles 43 along the longitudinal sides and receptacles 44along the ends 14 and 19 are formed. These receptacles are adapted toreceive the flange-like sections 35 and 4a which form the mechanicalsupport for the window 31. Numeral 39 designates a rubber gasket thatmay be used to secure a watertight pressure-proof fitting about thewindow 31 if that is desired. If this gasket is used it should be of amaterial having a dielectric constant the same as or closely approachingthat of the material composing the dielectric window 31 and the flangeportions 35 and 4d.

The window construction is such that its length and breadth correspondto the respective aperture dimensions. It has a thickness substantiallyequal to one-half wavelength in the dielectric material at the operatingfrequency. The grooved recesses 43 have a depth equal to the windowthickness and the groved recesses 44 along the end sections have a depthof one-quarter wavelength in the dielectric at the operating frequency.These grooved sections 43, when filled with the flange-like dielectricsections 35 havinga one-half wavelength dimension, appear as shortedhalf wave sections and present apparent low impedance surfaces at thejunction of the window 31 and the flange sections 35. Insofar as theradio waves approaching the window 31 are concerned it appearssubstantially as if the side walls 36 and 37 continued outwardly to meetthe flared surfaces 34. The grooved sections 44 having a quarterwavelength depth, when filled with the flange-like sections 4% having aquarter wave dimension, appear as shorted quarter wave sections andpresent apparent high impedance surfaces at the junction of the window31 and the flange sections and effectively reduce the spillover or leakof radio energy around the end surfaces.

Again referring to Figs. 1, 2 and 3, in operation the waves supplied bythe translation device 15 are conveyed by the wave guide 16 to thedirective horn 17 where they are radiated. The opening of the born 17 issubstantially aligned with the short focal line 20 but is depressedslightly below the line so that it is removed fro mthe useful field ofthe reflector 18 while at the same time it energizes the focal line 20.Because of the small spacing between the plates 36 and 37, the waveemitted by the horn 17, and propagated in the half pillbox between theplates 36, 37 and toward reflector 19, has a linear front in any planeparallel to, or containing, the short focal line 20. In the verticalplane perpendicular to the focal line 20, the aforesaid propagated wavehas a circular front, as indicated by the curved dotted lines 12. Hence,the wave propagated toward the reflector 19 has a conical wave front.This wave impinges upon the parabolic surface of the curved member ll fiy e direction arrows 13, and by this diseases surface the circular frontmentioned above is converted to a linear front. The wave reflected bythe reflector 19 and having a substantially linear front in the verticalplane mentioned above and any plane perpendicular thereto, and hencehaving a plane front extending parallel to the reference plane 33' andperpendicular to the axis 23, moves toward the main reflector 29 in adirection perpendicular to the latus rectum plane 38, and parallel tothe axis .5, until it reaches the outside of the window 31. Afterleaving the window 31 the wave front in the axial plane assumes acircular contour as indicated by the dot-dash lines 42 of Fig. 6. Thewave front in the vertical plane containing the focal line 23 andperpendicular to the short focal line remains linear, as indicated inperspective by line 41 of Fig. 6. Because the plane of the aperturecontaining the window 31 is at an angle a to the reference plane 33',that is, because the lengths of the diverse propagation paths extendingbetween the window 31 and the reference plane 33 vary linearly, the wavefront established by the half pill-box is conical, as shown in Fig. 6,the slope of the conical wave front being inclined at an angle of 2a tothe vertical plane 26 and at an angle or to the plane of the window 31.The window 31 and the longitudinal axis of the conical wave front aresubstantially coincident with the focal line 28. it should be rememberedthat this wave had a linear front parallel to the line 41 (Fig. 6)initially and this condition is retained as it progresses toward themain reflector 29, notwithstanding its circular front in the axial plane23. The portion of the main reflector having the surface 343 is aparabolic cylinder having a vertical focal line parallel to dimension46, and considering the vertical plane, it functions as a planereflector for the linear component or portion of the conical wave frontestablished by the line antenna element 18. Reflector 30 having aparabolic contour in its longitudinal plane transforms the circular wavefront, shown by the dot-dash lines 4-2 (Fig. 6) to a linear front. Thefinal outgoing wave therefore has a flat front since it has linearcharacteristics in perpendicular planes and assumes the character of apoint beam with the attendant high gain. Because the impinging wavefront 41 (Fig. 6) strikes the cylindrical parabolic surface 30, whichacts as a plane reflector in the vertical plane, at an angle of 20a tothe horizontal plane, and because the optical or mathematical axis 22.of this reflector is inclined at an angle or from the horizontal plane,the effective angle of incidence is a, as is the angle of reflection,which places the plane of maximum energy distribution in the horizontalplane with the flat wave front perpendicular to this plane. Because ofthe foregoing angular relations, the system is ideally suited forazimuthal scanning.

From the foregoing it will be noted that the shape of the horizontalplane characteristic of the beam is controlled by the parabolic contourof the main reflector 29 since the auxiliary reflector 1%; acts as aplane re flector in this plane. Similarly the shape of the verticalplane characteristic of the beam is essentially controlled by theparabolic contour of the reflector 18 since the main reflector 2? actsas a plane reflector in this plane over its surface 30. By properlycorrelating the dimensions of the two reflectors a wide choice of lobepatterns are available. In one tested embodiment wherein thelongitudinal dimension of the window 31 was slightly greater than thelength of the focal line 28 and the longitudinal dimesion 47 of the mainreflector was about 60 times as great as the length of the focal line20, a beam was obtained in which the width at the half power point inthe vertical plane was 5 times that of the width in the horizontal planeat the same power point.

It was previously stated, the lobe pattern in the vertical plane isessentially controlled by the parabolic contour of reflector 18 and itsrelation to the length of the focal '6 line 28 and this is true for lowelevational angles. However, some of the wave energy from the auxiliaryreflector l8 strikes the convex parabolic surface 3i) and as thetransverse axis of this section departs from the straight line axis ofthe cylindrical section in substantially as a square law function it isevident that this section does not act as a plane reflector along itstransverse axis and therefore the energy is reflected at an anglegreater than a and is accordingly directed above the horizontal plane.The exact contour of this section is selected by trial to produce thedesired energy distribution. As will be noted in the discussion of Fig.8 in one embodiment this surface was shaped so that its effect whencombined with the beam above referred to produced an asymmetrical lobepattern in which the upper half approximated a desired theoreticaldistributional pattern in accordance with the cosccant curve for uniformillumination of a certain desired target plane. From the foregoing it isevident that this system, in addition to being admirably suited forazimuthal searching produces a beam with cosecant coverage toexceptional high elevational angles and, therefore, is also admirablysuited for searching in a vertical plane perpendicular to the mainsearch plane. This result is accomplished through the use of relativelyeasily manufactured reflectors and by so placing the primary antenna 17and the auxiliary reflector 13 in a depressed position such that they donot impair the antenna distributional pattern through reflective orshadow effects and substantially eliminate frequency sensitivemismatches since a negligible amount of the radiated energy is reflectedback into the feed line 16.

Figs. 7, 8 and 9 indicate some measured results obtained on a particularantenna system employing one embodiment of the invention and having adesign frequency corresponding to a wavelength of 3.3 centimeters. Somerepresentative physical dimensions were as follows. Focal line 2% andspacing between side plates and 3? one-half inch. Window aperture Ellone-half inch wide by 6.84 inches high. Focal length L of the mainreflector 29 ten inches. Longitudinal axis thirty inches. Transverseaxes 45 and 46 respectively two and five and one-half inches. Angle atapproximately nine degrees. Set back" iii of the surface 39 as comparedto the surface 3% was one-half inch.

Referring to Fig. 7 the main lobe '70 and the minor lobes 72 are shownfor the horizontal plane. The main lobe is symmetrically disposed aboutits axis and has a width of substantially 2.6 degrees at its half powerpoint 71 corresponding to a decrease of 3 decibels from its peak value.it will be noted that the most closely associated minor lobes 72. are atleast 22 decibels below the peak of the major lobe 76. Although not hereshown the actual measurements indicated the next occurrence of minorlobes of sizable magnitude was at plus 25 degrees and minus 30 degreeswhere lobes 27 decibels and 28 decibels, respectively, below the peakvalue, were noted. All other measured lobes were below these levels. Theantenna gain of this system was found to be 27.7 decibels above that fora theoretical spherical antenna.

Fig. 8 indicates the energy distribution in a vertical plane for thedescribed system. The main lobe 75 has a width of approximately 16degrees at the half power point '76 corresponding to a decrease of 3decibels from its peak value. This width when comparec to the 2.6degrees at a similar point in the horizontal indicates a beam raving anaspect ratio greater than 6 to 1. That portion of the curve that wouldnormally show as minor lobes has been here grouped under reference nur-al '73 since they now form a part of the major lobe 72 by virtue of thereflective effect of the convex parabolic section 3% of the mainreflector 29. in this connection, it is interesting to note that thischanged contour 3d of the main reflector 29 gives the first indicationof its effect in this embodiment at about 17 degrees elevation asdesignated by the numeral and has no appreciable effect on the lowerhalf or down half of the pattern. Because of this action the major lobe75 is vertically asymmetrical about its axis in a horizontal plane andapproximates and lies above the curve 77 for all an es up to about 64degrees from the horizontal. Curve 77' represents the idealized curve ofenergy distribution suflicient to provide uniform scanning illuminationof a target plane such as would encompass a target first noted at 15,000yards range and 1,000 feet elevation and which approaches the antenna atconstant altitude until it is overhead. This is the so-called cosecantcurve or cosecant pattern. As the upper half of curve 75 closelyapproximates the curve 77 and remains constantly above that curve forall angles up to about 64 degrees of elevation above the horizontal itfollows that this system provides a beam with cosecant pattern suitablefor sky searching when mounted for azimuthal scanning.

Fig. 9 shows the extremely wide band characteristic of a system such asdescribed. Curve S1 indicates the level of standing waves as measured inthe wave guide 16 adjacent to the directive horn 17 for variousfrequencies corresponding to wave-lengths from 3.1 centimeters to 3.5centimeters. The impedance match of this system remains substantiallyunchanged over this wide band because of the substantial elimination ofthe reflection of radiated c 'y bacl; into the feed lin 16. This resultwas achieved it rough the double depression of the encrgizing sourcesbeneath t a flelds of radiation as was done in aligni" th directive horn17 with, but slightly beneath, .ne of reflector 19 and also bydepressing the line antenna beneath the scanning plane of the main ector29. it should be noted that the half pill-box or parallel plate lineelement 18 constitutes a wave guide and since the dielectric window Siis at an acute angle to wave propagation direction 23 or 24 in the halfpill-box l8, wave components reflected by the inner or outer surface ofthe window toward reflector 19 are not directed into the horn 17, thatis, the window angle is such that the reflected or feedback waveletsjust mentioned are not focussed on horn 17, whereby standing waves arenot established in guide 16. In addition to the elimination of frequencysensitive impedance mismatches the depression the reflector 13 makespossible the production of energy distribution patterns having minorlobcs suppressed to considerably greater extent than most prior artsystems.

Although the invention has been explained in connection with certain embdiments it should be understood that it is not to be so iited theretosince other apparatus may be employed in successfully practicing theinvention.

What is c imed is:

nation, a cylindrical parabolic reflector havr. in cc' me included in avertical plane and extending 'izontal plane containing the turn radioaction, a line an ially coincident with said focal plane.

2. l corn having a;

1 flector having ""ticn. a first cylindrical parabolic reflector asecond cylindrical parabolic rel plane perpendicular to the axialrcnector, a pair of parallel end plates attached to said secondreflector and forming therewith a rectangular rture, the plane of saidaperture forming an acute dihedral angle with the axial plane of saidsecond reflector.

3. ln an ant" angle, the angle between the horizontal plane and theaxial plane of said second reflector being twice the dihedral angleformed by the intersection of the plane of the aperture and the verticalplane, whereby the directive pattern of said antenna system is notdistorted by said second reflector.

4. A directive antenna system having in a given plane of action adirective pattern including one major lobe and at least one minor lobe,said system comprising a first and a second cylindrical parabolicreflector facing each other and having their axial planes at rightangles, the secondreflector being spaced from said plane of action, theplane bisecting the dihedral angle formed by the intersection of saidplane of action and the axial plane of said second or including amathematical axis of a parabolic segment of said first reflector,whereby the directive pattern of said antenna system is not distortedand tie intensity of said minor lobe is maintained at a minimum withrespect to the major lobe intensity.

5. In combination, a first reflector comprising plex surface, the upperportion of which is a convex surface such as would be described bypassing an are along a parabolic curve, the lower portion of which has asubstantially cylindrical parabolic shape and a focal line, said lowerportion constituting more than one half of the total reflector, a secondreflector having a substantially cylindrical parabolic shape, an axialplane, and a focal line perpendicular to the focal line of said firstreflector, 21 pair of parallel end plates attached to said secondreflector and forming therewith a rectangular aperture facing said firstreflector, the plane of said aperture being at an acute angle to theaxial plane of said second reflector and being located at substantiallythe focal line of said first reflector, whereby an asymmetricaldirective pattern having an aspect ratio of at least five to one issecured.

6. In combination, a cylindrical parabolic small. reflector having ashort focal line and an axial plane, a large reflector the greaterportion of which has a focal line of greater length than said firstmentioned focal line and the lesser portion of which presents a convexparabolic surface, a pair of parallel end plates attached to said smallreflector and forming therewith an antenna aperture locatedsubstantially coincident with the focal line of said large reflectorwhereby said small reflector illuminates said large reflector and thedirective pattern, in a plane containing the focal line of the largereflector and extending perpendicular to the short focal line of thesmall reflector, has a contour substantially in accordance with acosecaut curve.

7. A directivc scanning radio system having an energy distributionpattern at the half power point at least five times as wide in one planeas is its energy distribution pattern at the half power point in aperpendicularly related plane, said system comprising a main reflectorhaving a substantially convex parabolic surface in its upper portion anda cylindrical parabolic surface in its lower portion said lower portionexceeding said upper portion and having a focal line inclined at anacute angle with the vertical, a second cylindrical parabolic antennahaving a short focal line and an axial plane perpendicular to the focalline of said first reflector, a of parallel end plates attached to saidsecond reflector and forming therewith a rectangular aperture, saidaperture being located substantially coincident with the focal line ofsaid first-mentioned reflector, a directive wave guide antenna locatedat substantially the focal line of said second cylindrical parabolicantenna, and a translation device connected to said directive antenna,whereby in transmission the wave emitted by the primary antenna isconverted to a conically shaped wave front coincident with the focalline of said first-mentioned reflector, and said first reflectorconverts said conically shaped wave front to a plane wave front havingits energy distribution at least five times as concentrated in one planeas it is in a pera compendicularly related plane, and whereby inreception the converse operation obtains.

8. A directive reflector fcr the propagation of radio waves comprising acylindrical parabolic reflector having an axial plane and a short focalline, a pair of parallel end plates attached to said reflector andforming therewith an antenna aperture having given longitudinal andtransverse dimensions in a plane at an acute angle to said axial plane,a dielectric member for closing said aperture without substantiallyimpairing the radiating efflciency of said reflector, a clamping memberhaving suitable means for attachment to the reflector structure so as toform therewith a grooved recess for fastening said dielectric memberover said aperture, said grooved recesses having a depth ofsubstantially one half the length of the radio wave in the dielectricmaterial along the longitudinal sides and a depth of one fourth theaforesaid wavelength along the transverse ends of said aperture, saiddielectric member having a shape such that it extends one half of saidaforesaid wavelength beyond each longitudinal edge of said aperture andone fourth the aforesaid wavelength beyond each transverse end of saidaperture, each edge of said dielectric member being short-circuited bysaid clamping member, said dielectric member having a thickness ofsubstantially one half of said aforesaid wavelength throughout the areaof said member continuous to said aperture and having else where athickness of less than one half of the aforesaid wavelen th, wherebysaid dielectric member is apparently supported by low impedance elementsalong the longitudinal sides and high impedance elements along saidtrans verse ends of said antenna aperture.

9. A dielectric member for transmitting radio waves through an antennaaperture having given transverse and longitudinal dimensions, saidmember comprising a block of dielectric material having a mainconducting body portion and a secondary contiguous circumscribingportion in the shape of flange surfaces projecting from the sides andends of said main portion, said main body portion having substantiallythe same coincident transverse and longitudinal dimensions as saidaperture and in a plane perpendicular to the plane containing saidtrasverse and longitudinal dimensions a thickness dimensionsubstantially equal to one half the length of the radio wave in saiddielectric at the operating frequency, said secondary circumscribingcontiguous portion having a thickness dimension less than the thicknessof said main portion and extending outwardly from each longitudinal sideof said main body for a distance equal to substantially one half thelength of the wave in the dielectric at the operating frequency, wherebysaid conducting main portion may be supported along its longitudinalsides by apparent low impedance surfaces.

10. A dielectric member according to claim 9, said secondary contiguouscircumscribing portion extending outwardly from said main body alongeach transverse end section for a distance equal to substantially onefourth the length of the wave in the dielectric at the operatingfrequency, whereby said conducting main portion may be supported alongits transverse end sections by apparent high impedance surfaces.

11. A terminating member for a radio wave propagating antenna aperture,said member being composed of a dielectric material of complex shape,said member comprising a block section of suitable length and breadthdimensions for fitting contiguously inside of said aperture and having athickness substantially equal to one half of the wavelength of saidradio wave in said dielectric, and two flange-like sections each havinga thickness equal to less than one half of said wavelength and of thesame length as said block section, each of said flange sectionsextending outwardly from a side of said block section for a distanceequal to one half of said Wavelength measured along a line substantiallynormal to said side, whereby a minimum impedance change is presented tothe propagated radio wave in the desired plane of propagation and inapparent low impedance surface is presented to said wave in the plane ofthe juncture of said block section and flange sections when said flangesections are substantially everywhere in contact with the metallicsurface of said antenna member.

12. A terminating member for a radio wave propagating antenna aperture,said member being composed of a dielectric material and having a complexshape, said member comprising a block section having a thickness equalto one half the length of the operating radio wave said dielectricmaterial and a length and breadth suitable for fitting contiguouslyinside of said aperture, two iiangedike sections each having a thicknessof less than one half of said wavelength and having another dimensionequal to the width of said block section, each of said flange sectionsextending outwardly from an end of said block section for a distanceequal to one fourth of said wavelength along a line substantially normalto said end, whereby a minimum impedance change is presented to thepropagated radio wave in the desired plane of propagation and anapparent high impedance surface in the plane of the juncture of saidblock and flange sections is presented to said radio wave when thelateral faces of said flange sections are substantially everywhere incontact with a metallic surface.

13. A terminating member for a radio wave propagating antenna aperture,said member being composed of a dielectric material of complex shape,said member comprising a first section having a substantially uniformthickness equal to one half of the length of the operating radio wave inthe dielectric material and having suitable length and breadth forcontiguous fit inside of said aperture, and a second section of athickness equal to less than one half of the length of said Wave in saiddielectric, said second section bounding the two ends and two sides ofsaid first section and extending outwardly from said sides a distanceequal to one half 4 of said wavelength and extending outwardly from saidends a distance equal to one fourth of said wavelength when measuredalong lines substantially normal to said sides and ends of said firstsection, whereby a minimum impedance change is presented to saidpropagated radio wave in the desired plane of propagation, apparent lowimpedance surfaces are presented to said radio wave in the planes ofjuncture of said first and second sections along the lengthwisedimensions and apparent high impedance surfaces are presented to saidradio wave in the planes of juncture of said first and second sectionsparallel to said end sections when the lateral faces of said secondsection are substantially everywhere in contact with a metallic surface.

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